1. Field of the Invention
The invention relates to an impeller for a water jet pump of a watercraft, and more particularly to an attaching structure of a pump shaft side of the impeller and a propeller shaft side thereof.
2. Description of the Related Art
A watercraft which is propelled by a water jet pump, for example, a personal watercraft (which is also referred to as a "PWC") is configured to suck water (including sea water) through a water intake hole provided on a bottom of a hull. The water jet pump pressurizes the water and ejects the water rearward from the hull through an outlet port of the water jet pump, thereby advancing the PWC.
The water jet pump structure includes an impeller, which is rotatably provided in a pump casing, the inner periphery of which is circular-shaped. The impeller has a base portion shaped rather like a cone with its tip cut off (a frustum of a cone), that is, a "bullet shape," with blades provided on the outer periphery of the base portion. That is to say, the outer shape of the base portion is smaller on a tip side (upstream, or propeller shaft, side) and gradually becomes larger toward a more rearward portion. The blades (fins) are projected with a pitch angle to forcibly discharge water rearward on an outer peripheral portion of the base portion.
The tip portion of the base of the impeller is connected to a rear end of the propeller shaft on the engine (upstream) side so as to be rotated integrally with the propeller shaft. A rear (downstream) end of the impeller is connected to a tip portion of a pump shaft, so as to be rotated with the propeller shaft and the pump shaft.
The water sucking (front, or upstream) side of the water jet pump is connected to a water sucking (intake) passage having a water intake formed on the bottom of the hull. A pump nozzle, incorporating an outlet port of the water jet pump, has a cross-sectional area gradually reduced rearward. Consequently, water pressurized by the impeller is discharged at a high speed through the outlet port of the pump nozzle positioned at the rear end of the watercraft. Thus, the watercraft can advance by means of a water jet reaction force.
The portion of the impeller connecting to the propeller shaft comprises a spline hole formed in the tip portion of the impeller, and accommodates spline teeth formed on a rear-end portion of the propeller shaft such that the propeller shaft can be spline-connected to the spline hole. A connecting portion of the impeller to the pump shaft comprises a female screw-threaded hole formed in a rear end portion of the impeller. A male screw-threaded portion provided on a tip portion of the pump shaft is provided such that it can be screwed to the female screw hole.
In the prior art, the following techniques, described with reference to FIGS. 5 and 6, are known for providing connecting portions of the propeller shaft (upstream) side, and the pump shaft (downstream) side of an impeller.
More specifically, an impeller 1 shown in FIG. 5 which has been described in Japanese Utility Model No. 2513811, has a spline hole 1b and a female screw hole 1d provided on an impeller 1 itself, respectively. The spline hole 1b is provided by forming spline grooves one by one at predetermined intervals on the inner peripheral face of a prepared hole that is drilled before, by using a machine tool which is referred to as a "slotter." The female screw hole 1d is provided by forming a female screw 1m on the inner peripheral face of a prepared hole that is drilled, by using a machine tool which is referred to as a "tapper" or the like. An impeller manufactured by such a technique requires considerable working time. A special jig having a complicated shape, or the like, for setting (holding) is required. Consequently, the manufacturing cost of an impeller has increased.
With a structure such as is shown in FIG. 6, a spline hole 1b is formed beforehand in a cylindrical member 1Y by using a "broaching machine". A fitting hole 1y for fitting the cylindrical member 1Y therein is formed on the tip side portion of the impeller 1 and a prepared hole for a female screw is formed in the rear end of the impeller. A female screw-thread 1m is applied to the prepared hole for forming the female screw by means of the "tapper", and the cylindrical member 1Y is then fitted in the fitting hole 1y on the tip side portion of the impeller 1.
The impeller manufactured by such a technique is made with greater working efficiency than the impeller shown in FIG. 5. However, an outside diameter d.sub.1 of the tip portion of the impeller is increased, so that the pump efficiency of the impeller is reduced.